This invention relates to variable coupling optical waveguide couplers, and is an improvement in, or modification of, the couplers described in U.S. Pat. No. 3,976,358 issued Aug. 24, 1976 and assigned to the common assignee of the instant invention.
The aforementioned application describes couplers fabricated in semiconductive material in which the coupling between a pair of optical waveguides may be altered by reverse biasing at least one p-n junction which is located so that under conditions of reverse bias a depletion region is established which, on the n-type side, extends into or through one of the waveguides or the region between them. A problem in the design of such couplers is the choice of suitable carrier concentrations to provide adequate optical guidance in the lateral direction. Optical guidance in one direction is provided by two heterojunctions, but in the lateral direction guidance is provided by refractive index differences between regions of relatively high and relatively low free electron concentration. A large difference in concentration provides strong guidance, but has the disadvantage that a high free carrier concentration produces undesirably high optical loss. In the present invention this problem is circumvented by adopting an alternative way of achieving optical guidance in the lateral direction. If significant use is to be made of refractive index changes brought about by free carrier removal, it is still however necessary to select a doping level which is high enough to produce an adequate change of refractive index when the depletion region is established, but yet is not so high as to make the depletion region very thin compared with the thickness of the waveguides.